// Here's my custom control

$widget->add_control(
    'wpf_fields_map',
    [
        'label' => sprintf( __( '%s Field Mapping', 'elementor-pro' ), wp_fusion()->crm->name ),
        'type' => Fields_Map::CONTROL_TYPE,
        'separator' => 'before',
        'fields' => [
            [
                'name' => 'remote_id',
                'type' => Controls_Manager::HIDDEN,
            ],
            [
                'name' => 'local_id',
                'type' => Controls_Manager::SELECT,
            ],
        ],
        'default'   => $this->get_fields(),
    ]
);

// Need to merge in new fields....

$settings = $widget->get_settings();

if ( ! empty( $settings['wpf_fields_map'] ) ) {

    $fields         = $this->get_fields();
    $new_fields_map = array_merge( $fields, $settings['wpf_fields_map'] );
    $widget->set_settings( 'wpf_fields_map', $new_fields_map );

}